1. Field of the Invention
The present invention relates to wireless communication. More particularly, the present invention relates to spectrum collaboration between multiple overlapping Wireless Regional Area Networks (WRANs).
2. Description of Related Art
At present, in communication protocols, such as those defined in the Institute of Electrical and Electronics Engineers (IEEE) 802.22 standard, etc., no regulation or description is given on how to effectively address the problem of channel collision between two or more overlapping WRANs by using collaboration.
For a WRAN applying a cognitive radio technique, research is being conducted on how to better use limited idle frequency bands to implement area access communication.
When each Consumer Premise Equipment (CPE) in a WRAN performs in-band spectrum sensing, the access network spends a long period detecting frequency bands that are occupied by respective authorized users.
An example of conventional techniques are techniques defined in the IEEE P802.22/D0.2, Draft Standard for Wireless Regional Area Networks Part 22: Cognitive Wireless RAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: Policies and procedures for operation in the TV Bands, November 2006, the entire disclosure of which is hereby incorporated by reference. In conventional techniques, a frequency hopping operation scheme is proposed for a WRAN based on dynamic frequency hopping. In the frequency hopping operation scheme, periodic frequency hopping between different idle channels is used to reduce the quiet period for in-band spectrum sensing. FIG. 1 illustrates a dynamic frequency hopping process of multiple WRANs according to the conventional art. More specifically, FIG. 1 illustrates flexible frequency hopping of WRAN-1 104 and WRAN-2 105 between CHannel (CH) A 101, CH B 102 and CH C 103. WRAN-1 104 operates in the first period of CH A 101. In the second operation period, WRAN-1 104 hops to the idle CH B 102 to establish communications. During the third operation period, WRAN-1 104 hops to the idle CH C 103. Similarly, WRAN-2 105 sequentially hops from CH C 103 to CH A 101 and then to CH B 102 during the three operation periods. Therefore, not only may WRAN-1 104 and WRAN-2 105 operate in idle channels, but also the CPEs may implement normal spectrum sensing during the quiet periods of the channels.
FIG. 2 illustrates a dynamic frequency hopping operation in a WRAN according to the conventional art. More specifically, FIG. 2 illustrates a process by which WRAN 201 operates during three periods, such as the initial spectrum sensing stage 202 and the two operation stages 203 and 204. Initially, the CPEs in WRAN 201 implement the initial spectrum sensing process 202 to detect the idle frequency band and the valid time 205 of CH A for the normal operation of the system. During the second operation period, WRAN 201 hops to CH A to transmit data and at the same time implements spectrum sensing 203 for CH ([0,A−n],[A+n,N]) to obtain the valid time 206 of CH B. During the third operation period, WRAN 201 hops to CH B and at the same time implements spectrum sensing 204 for CH ([0,B−n],[B+n,N]). Here, N denotes the total number of channels to be detected, and n denotes the width of a guard band.
The issue of how to address the spectrum collision between multiple WRANs using dynamic frequency hopping is being researched. In a conventional solution, the control center of WRAN-1 that has first detected the idle CH A announces, by transmitting signaling to other WRANs, that it has pre-occupied the detected idle channel, and at the same time the control center of WRAN-1 monitors the announcement broadcast information from the other WRANs. If there is no broadcast information on the pre-occupation of CH A, or a pre-occupation announcement from the other systems occurs later than the pre-occupation announcement of WRAN-1, then WRAN-1 hops to CH A in the next period. A scenario that occurs when the above described collision-avoiding-based solution is adopted to address the spectrum collision in WRANs is hereafter described with reference to FIG. 3. FIG. 3 illustrates channel collision between two overlapping WRANs according to the conventional art.
During the first operation period, WRAN-1 301 detects the valid time 303 of CH A and WRAN-2 302 detects the valid time 304 of CH D. Then WRAN-1 301 and WRAN-2 302 respectively hop to CH A and CH D during the second operation period. In the second operation period, the following two cases may occur:                (1) both WRAN-1 301 and WRAN-2 302 have only detected that CH B is in an idle state, and they have obtained different valid times, i.e., valid time 305 for CH B by WRAN-1 and valid time 306 for CH B by WRAN-2;        (2) only WRAN-1 301 detects that CH B is idle; meanwhile, WRAN-2 detects that CH B and CH C are idle. However, the valid time 307 for CH C exceeds a maximum delay limit of WRAN-2.        
Accordingly, the collision-avoiding solution based on a contention mechanism is not adequate to address the above two cases of spectrum collision.
Among conventional techniques, a frequency-division-based spectrum collaboration method is proposed to address channel collision in the above two cases. However, this method has deficiencies. Since all WRANs participating in frequency collaboration are in an operational state, new problems may occur as follows:                (1) Significant bandwidth resources are spent in producing guard bands; especially in the case that the number of the participating WRANs increases, the Base Station (BS) transmission power increases and the overlapping areas among cells increase, therefore an increasing amount of spectrum resources have to be used to generate guard bands to address interferences between the cells;        (2) Since all WRANs are in operation during all operation periods, much more transmission power is wasted.        
Therefore, a need exists for a method for spectrum collaboration between multiple overlapping WRANs.